1. Field of the Invention
The present invention relates to a method of controlling print density in thermal transfer printing and, more specifically, to a method of accurately controlling density grade in a dye diffusion type thermal transfer printing.
2. Description of the Background Art
FIG. 1 schematically shows an example of a main portion of a thermal transfer printer. In such a thermal transfer printer as shown in FIG. 1, an ink sheet 2 drawn from an ink sheet supplying roll 1 is overlapped with a sheet of recording paper 5 between a thermal head 3 and a platen roller 4. The thermal head 3 comprises a plurality of heating elements 3a arranged in a line in a direction orthogonal to the sheet of the drawing. Energy pulses corresponding to desired print density are applied to the heating elements 3a, whereby dye diffusion type ink on the ink sheet 2 is transferred on the sheet of the recording paper 5 and thermal transfer printing is effected. These heating elements 3a are formed by register elements, and the energy pulses are applied as electric energy pulses. The amount of energy of each energy pulse can be controlled by changing the width of pulse current or by the changing voltage level of pulse current. After thermal transfer printing, the ink sheet 2 is separated from the printed sheet of the recording paper 5 along a separation board 6 to be wound up around an ink sheet winding roll 7. The sheet of recording paper 5 is moved at the same speed as the ink sheet 2 by means of a capstan roller 8 and a capstan pressure roller 9.
Now, when printing is continued by repeatedly applying energy pulses of a predetermined energy to the heating elements 3apart of the heat generated by the heating elements 3a is stored in the thermal head 3. Consequently, the temperature of the thermal head 3 gradually increases, and finally it reaches a steady temperature. At this time, temperature gradient in the thermal head 3 is at a steady state, and this state is hereinafter referred to as the steady temperature state of the thermal head 3. When energy pulses are applied to the heating elements 3a with the temperature of the thermal head 3 already increased, the actual printed density becomes higher than when the same energy pulses are applied to the heating elements 3a with the temperature of the thermal head 3 being low.
FIG. 2 is a cross sectional view of a thermal head having a temperature detector. This thermal head 3 includes a plurality of heating elements 3a on the lower side, and a temperature detector 3b including a thermister on the upper side.
FIG. 3 is a graph showing relation between energy pulses determined by using the thermal head of FIG. 2 and the actual printed density, dependent on the steady temperature of the thermal head 3 (temperature of the thermal head 3 when temperature gradient from the heating elements 3a to the temperature detector 3b is at the steady state). In the graph, the abscissa represents thermal energy (mJ/dot) applied to one heating element corresponding to one print dot, and the ordinate represents actual printed density represented by OD (Optical Density) value. Curves represented by a solid line, a dotted line and a chain dotted line correspond to the steady temperature of 30.degree. C., 40.degree. C. and 50.degree. C., respectively, of the thermal head 3 measured by the temperature detector 3b. The maximum available density (e.g., 2.00) depends on a recording ink, while the minimum density (e.g., 0.08) depends on the recording paper.
As will be understood from the graph of FIG. 3, the actual printed density is influenced by the temperature of the thermal head 3, and it is not always proportional to the pulse energy applied to the heating elements 3a. Accordingly, adjustment of energy applied to the heating element 3a by means of a head driver referring to a density table prepared based on the graph such as shown in FIG. 3 has been proposed. However, since such a density table is formed assuming that the thermal head 3 is at the steady temperature state, the actual printed density at the start of printing when the thermal head 3 is at non-steady state or the actual printed density when low density printing is abruptly changed to high density printing tends to be lower than the desired density. Conversely, the actual printed density when high density printing is abruptly changed to low density printing tends to be higher than the desired density. Therefore, correction to increase the density and correction to decrease the density are necessary to obtain the desired printed density.